It is September 19 1982, the day after the Lebanese Forces militia left Beirut's Palestinian camps after a 38-hour orgy of killing, and it is finally possible to see what the Israeli soldiers surrounding the camps claimed they had been unable to see. Streets carpeted with bodies. Men, women and children shot and hacked to death. Pregnant women eviscerated. In Christian East Beirut, Israel's chief of staff, Lieutenant General Rafael Eitan, the commander of the Northern District, Major General Amir Drori, and a senior Mossad officer, Menahem Navot, codenamed Mr R, meet the deputy chief of staff of the Lebanese Forces, Antoine Breidi - "Toto" - and Joseph abu Khalil, the man who made the first contact with the Israelis in March 1976. What ensues is a cynical damage-limitation conference in which senior officers of the Israeli Defence Forces utter not one word of reproach for a massacre in which mili tiamen trained, armed and sent into the camps by them killed at least 900 defenceless civilians.

Gen Eitan: "Everybody points an accusing finger at Israel and the outcome might be that the IDF will be forced to withdraw from Beirut. Therefore some of you have to explain the subject and immediately. The formula should be that they [the Lebanese Forces] took part in an assignment and that whatever occurred was out of their control."

Gen Drori: "On this occasion you should mention also what happened at Damour [a Christian village where fighters including Palestinians killed 200 civilians in 1976]. Also to mention the fact that this is not your policy. You could mention that in the places that they entered there were battles between rival sides inside the camps and not only with the Phalangists [the LF's political umbrella]."

Abu Khalil: "... You tell us and we will carry it out."

And so it goes on - a web of evasions and untruths concocted by the IDF, which sent 200 Lebanese militiamen into Sabra and Shatila on September 16 to "mop up" 2,000 "terrorists" who Ariel Sharon, then Israel's defence minister, claimed had remained there after the PLO's evacuation from Beirut. It is an encounter that shows the intimacy between the IDF and the LF, even after the massacre, and the virtual incorporation of the LF into the IDF structure.

Two almost identical reports of this meeting - one identified as "a transcript of a conversation recorded by an aide to the commander of the Northern District"; the other as "Minutes of Mossad (4222) of a meeting between Israeli chief of staff and Gen Drori with Toto" - are among a stack of documents delivered to lawyers seeking to bring Sharon, now Israel's prime minister, to trial in Belgium for war crimes committed in Lebanon 19 years ago when he had overall responsibility for the IDF.

The documents, exclusively obtained by the Guardian, cover the period between June and November 1982 - from a meeting in which "the cabinet has decided to have the Lebanese army and the Phalangists participate in the entering of Beirut" to the testimony to Israel's Kahan commission of inquiry of a senior military intelligence officer, Colonel Elkana Harnof. Some are in Hebrew; others in English. Michael Verhaeghe, one of three lawyers representing the plaintiffs in the case against Sharon, has little doubt about the documents' authenticity. They arrived anonymously in June, within 10 days of the suit being lodged under legislation that allows Belgium to prosecute foreigners for war crimes, wherever they were committed.

"The documents give a very detailed account of a number of events which would be very difficult to fabricate - especially in that very short period of time," says Verhaeghe. Investigations by the Guardian in Israel and Lebanon have confirmed the identity of the intelligence officers named in the documents as well as the dates, times and locations of some of the meetings, those who attended them and some of their content. The typescript of the Hebrew documents matches that used at the time of Kahan. And the voices of many of the protagonists are unmistakable - among them the courtly Pierre Gemayel, patriarch of the Gemayel family, and Sharon, referred to throughout as DM.

Thus, from minutes of a meeting on August 21 at Gemayel's home in Bikfaya: Pierre: "I visited Israel several times. I was very impressed." DM: "How to create power and how to convey its presence is the great test. We were 18 million, six million were exterminated... The use of power is what I want to discuss with you."

The lawyers say the documents' importance lies in recurring evidence that the IDF had "command responsibility" for the Lebanese Forces before, during and after the massacre. Thus, according to a summary of a meeting in which "the capture of Beirut" was discussed with LF leaders on July 13, Gen Eitan "explained that the IDF would provide all the necessary support: artillery, air etc as if they were regular IDF units".

"Under the established law of command responsibility - also known as indirect responsibility - this is watertight evidence of the conscious and effective chain of command," says Chibli Mallat, one of Verhaeghe's colleagues.

In February 1983, the Kahan commission found that no Israeli was "directly responsible" for the massacre, but determined that Sharon bore "personal responsibility". It ruled that he was negligent in ignoring the possibility of bloodshed in the camps following the assassination of the Lebanese Forces' leader, president-elect Bashir Gemayel, on September 14 - a massacre that Sharon publicly, and erroneously, blamed on Palestinians. Sharon resigned his defence portfolio, but stayed in the cabinet.

In Brussels today an appeals court will meet in closed session to decide whether to put Israel's prime minister on trial. Sharon's lawyers will argue that he has immunity as a head of government; that he is a victim of double jeopardy after the Kahan inquiry; and that the Belgian law cannot be used retroactively - claims that Mallat and his colleagues dismiss. Only if the appeals court rules for the plaintiffs will the documents be introduced to a court, obliging Sharon's lawyers either to acknowledge them or to produce others that refute them. Only then will a court hear of Sharon's early insistence that the Lebanese Forces "clean" the camps, despite their known proclivity for murder and rape.

Thus, even as the first PLO fighters left Beirut on August 21, Sharon met Bashir and Pierre Gemayel to demand a new strike against the Palestinian presence in Lebanon. Minutes of the meeting quote Sharon as saying: "A question was raised before, what would happen to the Palestinian camps once the terrorists withdraw... You've got to act... So that there be no terrorists you've got to clean the camps." Pierre Gemayel prevaricated: "We are in the midst of a political process of presidential elections... Bashir is the nominee... It is very important that calm is kept." Sharon insisted: "What would you do about the camps?" Bashir: "We are planning a real zoo."

In his testimony to Kahan, Sharon claimed that no one imagined the Lebanese Forces would carry out a massacre in the camps. This claim is contradicted by numerous testimonies in the documents in Belgium - among them Sharon's own complaint to Bashir Gemayel, minuted 10 weeks before the massacre, that "it is incumbent that we prevent several ugly things which have occurred - murders, rapes and stealing by some of your men". In the same month, in a meeting with American diplomats at the home of Johnny Abdo, Lebanon's military intelligence chief, Sharon proposed that the PLO fighters in Beirut be given "refuge" in Israel. "Although we are at a friend's house," he said, according to the report of the meeting, "rest assured that they would be more secure in our hands!"

Verhaeghe's documents show that this belief was shared by top intelligence officials identified in a secret part of the Kahan report - Appendix B. Kahan said Appendix B would not be published for reasons of national security. The lawyers believe the documents referring to these officers must come from Appendix B, but do not know whether the entire file is from Appendix B.

Echoing Sharon's concerns, according to excerpts from testimony to Kahan on October 22, Mossad chief Yitzhak Hoffi says the Phalangists "talk about solving the Palestinian problem with a hand gesture whose meaning is physical elimination... I don't think anybody had any doubts about this... They raised the issue of Lebanon being unable to survive as long as this size of population existed there." Similarly, Col Harnof, in a summary of his testimony a month later: "It was possible to surmise from contacts with the Phalange leaders what were their intentions towards the Palestinians: 'Sabra would become a zoo and Shatila Beirut's parking place' ... When they participated in actions east of Bahamdoun [when they operated against the Druze] they ran straight to the villages and committed massacres."

But the clearest indication of how the Lebanese Forces might solve the "demographic problem" was given by Bashir Gemayel himself in a meeting with Menachem Navot. In one account of this meeting, Bashir "adds that it is possible that in this context they will need several Dir Yassins" - a reference to the Palestinian village where 254 villagers were massacred in April 1948, in the most spectacular single attack in the conquest of Palestine.

In June this year, the first case involving the exercise of universal jurisdiction in Belgium resulted in the conviction of four Rwandans for war crimes committed in 1994. Mallat and his colleagues say they are determined to press for a similar result despite the accusations being levelled against them - among them anti-semitism, animosity and hatred. They say their starting point is not the criminal but the crime.

"I have a very profound belief that it is difficult to have peace in the Middle East without minimal accountability, certainly for the largest crimes," says Mallat. "We need a day of reckoning for the outstanding crime against humanity committed in Sabra and Shatila."